High power density fuel cells have long been desired.
Existing fuel cells generally are a stacked assembly of individual fuel cells, with each cell producing high current at low voltage. The typical cell construction involves reactant distribution and current collection devices brought into contact with a layered electrochemical assembly consisting of a gas diffusion layer, a first catalyst layer, an electrolyte layer, a second catalyst layer and a second gas diffusion layer. With the exception of high temperature fuel cells, such as molten carbonate cells, most proton exchange membrane, direct methanol, solid oxide or alkaline fuel cells have a layered planar structure where the layers are first formed as distinct components and then assembled into a functional fuel cell stack by placing the layers in contact with each other.
One major problem with the layered planar structure fuel cell has been that the layers must be held in intimate electrical contact with each other, which if intimate contact does not occur the internal resistance of the stack increases, which decreases the overall efficiency of the fuel cell.
A second problem with the layered planar structured fuel cell has been to maintain consistent contact between the layers for sealing and ensuring correct flows of reactants and coolants in the inner recesses of the layer structured fuel cell. Also if the overall area of the cell becomes too large then there are difficulties creating the contacting forces needed to maintain the correct fluid flow distribution of reactant gases over the electrolyte surface.
Existing devices also have the feature that, with the layered planar structure fuel cell since both fuel and oxidant are required to flow within the plane of the layered planar structured fuel cell, at least 4 and up to 10 but typically 8 distinct layers have been required to form a workable cell, typically a first flowfield, a first gas diffusion layer, a first catalyst layer, a first electrolyte layer, a second catalyst layer, a second gas diffusion layer, a second flowfield layer and a separator. These layers are usually manufactured into separate fuel cell components and then the layers are brought into contact with each other to form a fuel cell stack. When contacting the layers care must be taken to allow gas diffusion within the layers while preventing gas leaking from the assembled fuel cell stack. Furthermore, all electrical current produced by the fuel cells in the stack must pass through each layer in the stack, relying on the simple contacting of distinct layers to provide an electrically conductive path. As a result, both sealing and conductivity require the assembled stack to be clamped together with significant force in order to activate perimeter seals and reduce internal contact resistance.
The manufacture of the layers for existing fuel cell configurations is often expensive and difficult. The bipolar plates, which serve as oxidant and fuel flowfields as well as the separator are often constructed from graphite which is difficult to machine, adding significant cost to the fuel cell stack. The membrane electrode assembly (MEA) is usually constructed by first coating a solid polymer electrolyte with catalyst on either side and then pressing gas diffusion electrodes onto the electrolyte. The fuel cell assembly requires multiple individual bipolar plates and membrane electrode assemblies to be connected together in a serial manner. Usually discrete seals must be attached between neighbouring bipolar plates and membrane electrode assemblies and the whole stack of sealed bipolar and MEA layers must be held together under considerable compressive force.
A need has existed to develop alternative fuel cell designs that do not perpetuate the approach of assembling discrete layers in a serial manner. One way to meet this need is to build fuel cells using a micro-structured approach wherein microfabrication techniques and nano-structured materials can be combined to create novel devices not subject to the problems commonly associated with conventional fuel cell designs. The application of microscale techniques to fuel cells has a number of distinct advantages. Specifically, the potential for increased power density due to thinner layers and novel geometries, improved heat and mass transfer, improved and/or more precise catalyst utilization and reduced losses with shorter conductive path lengths will all make fuel cells more efficient and enable higher volumetric power densities. The opportunity to include ancillary systems into the fuel cell design and the potential for new applications to emerge present even more potential benefits.
A need has existed for a micro fuel cell capable of low cost manufacturing because of having fewer parts than the layered planar structure fuel cell.
A need has existed for a micro fuel cell having the ability to utilize a wide variety of electrolytes.
A need has existed for a micro fuel cell, which has substantially reduced contact resistance within the fuel cell.
A number of prior inventions have used microscale-manufacturing techniques with fuel cells. U.S. Pat. No. 5,861,221 presents a ‘membrane strip’ containing a number of conventional MEAs connected to each other in series by connecting the edge of the negative electrode of one MEA to the edge of the positive electrode of the next MEA. Two configurations are considered. The first constructs the ‘membrane strip’ by placing the MEAs together in a step-like configuration. The second constructs the ‘membrane strip’ by combining MEAs end-to-end with electrically conductive regions between them that connect the cells in series. In some follow-up work (U.S. Pat. No. 5,925,477) the same inventors incorporate a shunt between the electrodes to improve the electrical conductivity of the cell. The MEA's themselves are of conventional layered structure design, and the overall edge collected assembly continues to rely on conventional seals between neighbouring MEA's.
U.S. Pat. No. 5,631,099 and U.S. Pat. No. 5,759,721 use similar series connection concepts but apply a number of other microscale techniques to the fuel cell design. By doing so multiple fuel cells are formed within a single structure simultaneously. The fuel cells themselves still reside as layered planar devices mounted onto a carrier, with interconnection between neighbouring fuel cells requiring a penetration of the carrier layer. Most of the techniques discussed in these patents relate to the creation of methanol tolerant catalysts and the application of palladium layers to the catalyst to prevent methanol crossover within the cells.
WO 01/95406 describes a single membrane device that is segmented to create multiple MEA structures. Complex bipolar plates that are difficult to manufacture provide both fuel and oxidants to both sides of the MEA layer. U.S. Pat. No. 6,127,058 describes a similar structure, but instead of complex manifolding of reactant gases, only one reactant is supplied to either side of the MEA layer. Series interconnection of the fuel cells formed within the single MEA layer is achieved through external current collectors arranged around the perimeter of the device providing electrical connection from the top of the MEA layer to the bottom of the MEA layer. Such perimeter electrical connections are inefficient.
Some prior art fuel cells attempt to reduce size and fabrication costs by applying microfabrication techniques. For example the Case Western Reserve University device forms multiple fuel cells on a carrier substrate using thin layer processes similar to those used in printing and semiconductor fabrication (Wainwright et al. “A microfabricated Hydrogen/Air Fuel Cell” 195 Meeting of the Electrochemical Society, Seattle, Wash., 1999). In these designs the fuel cells remain of conventional planar design, with the exception that the fuel cells are built-up on a base substrate. The cathodes must be formed on top of the planar electrolytes and then must be connected to neighbouring anodes with an explicit interconnect.
All the cells presented above use current collection on the edge of the electrode. This significantly increases the internal cell resistance of these cells. Each of these cells are also based on solid polymer electrolytes as this is the only electrolyte that allows for easy manufacture. Furthermore, all of the cells presented above achieve a micro fuel cell design by forming multiple fuel cells within a single electrolyte plane.
The concept of using non-planar electrolytes has been considered in the past. GB 2,339,058 presents a fuel cell with an undulating electrolyte layer. In this configuration a conventional layered MEA is constructed in an undulating fashion. This MEA is placed between bipolar plates. This design increases the active area that can be packed into a given volume. However, this design still relies on the expensive and complicated layered structure with explicit seals and requires compressive force to maintain internal electrical contact and sealing. JP 50903/1996 presents a solid polymer fuel cell having generally planar separators with alternating protruding parts serving to clamp a power generation element (apparently an MEA) into a non-planar but piecewise linear shape. As with GB 2,339,058, this document continues to rely on the expensive and complicated layered structure but this design also puts undue stress on the MEA by forcing it into a non-planar arrangement using the separator plates.
In addition to non-planar designs, some prior art presents tubular configurations. U.S. Pat. No. 6,060,188 presents a cylindrical fuel cell with a single MEA layer formed into a cylinder. Fuel or oxidant is delivered to the interior recess of the cylinder with the other reactant delivered on the exterior. Within this design, each cylindrical structure creates a single cell, with current flowing through the annular cylindrical wall that is the fuel cell. A method of providing series electrical interconnection between fuel cells or of sealing individual fuel cells is not disclosed. This design is reminiscent of tubular designs for solid oxide fuel cells that are well known.
A need has existed to develop fuel cell topologies or fuel cell architectures that allow increased active areas to be included in the same volume, i.e. higher density of active areas. This will allow fuel cells to be optimized in a manner different than being pursued by most fuel cell developers today.